The experimental data on O2 absorption using reflected sunlight and a passive Fabry-Perot technique are presented. The atmosphere's irradiance measurements are an important tool for the remote sensing study. In this work we focus on the O2 A band (759-771 nm) composed of about 300 absorption lines, which vary in strength and width according to pressure and temperature. We performed measurements using solid Fabry-Perot etalons with different FSR and two different pre-filters. The first pre-filter selects a spectral range around 763 nm which is between the P and R branches, where the absorption coefficient is insensitive to temperature, but is sensitive to pressure changes and therefore to the variations in the O2 column. The second pre-filter is selecting several absorption bands between 765 and 770 nm, which are more sensitive to temperature changes.
The optical setup consists of two channels. Channel one measure the total reflected light, whereas channel two uses a solid substrate Fabry-Perot etalon to restrict measurement to light in the O2 absorption bands. The ratio of the intensities detected by the two channels is sensitive to O2 pressure change or temperature change depending of the spectral region and is a function of the air mass, solar zenith angle and altitude. The experimental data presented shows excellent agreement with our theoretical expectations. They were recorded at different gas pressures and temperatures, and also at various weather conditions. The goal of the experiment is to demonstrate that variations of the column density of the O2 can be detected using a solid Fabry-Perot etalon. Results can be used for normalization of the other trace gases column densities, (to measure CO2 column density) because the Oxygen is well mixed throughout most of the atmosphere (to an altitude of about 100 km) and can help to interpret the influence of scattering from aerosol and clouds, polarization of the reflected light, and the reflection properties of the surface. Some of the major advantages of our optical setup are its compactness, high sensitivity, high signal-to-noise ratio and stability.